1. Field of the Invention
The present invention relates to an electroabsorption optical intensity modulator.
2. Description of the Related Art
In a high speed optical communication system for long distance, semiconductor laser diodes have been broadly used to adopt a direct modulation method. In this system, however, a wavelength chirping phenomenon is generated during modulation, so that the waveform of a transmitted light signal on an optical fiber is deteriorated. This chirping phenomenon becomes critical as the transmission rate becomes larger and the transmission distance becomes longer. Particularly, in a communication system using 1.3 .mu.m zero dispersion fibers, even when a light source of a 1.55 .mu.m band having a low loss of fiber transmission is used to increase the transmission distance, this distance is limited by the dispersion limit due to the chirping phenomenon.
In order to reduce the chirping phenomenon, an external modulation system where an external optical modulator is used has been developed. As the external optical modulator, there are a dielectric modulator using LiNbO.sub.3 or the like and a semiconductor modulator using InP, GaAs or the like. The semiconductor modulator has an advantage in that the semiconductor modulator can be integrated with other optical elements such as a semiconductor laser diode and an optical amplifier and other electronic circuits such as a field effect transistor (FET) circuit, and also, the semiconductor modulator can be small in size and low in power supply voltage.
The semiconductor modulator is further divided into an electroabsorption optical intensity modulator and a Mach-Zendor type modulator. In the electroabsorption optical intensity modulator, the Franz-Keldysh effect for a bulk semiconductor or the quantum confined stark effect for multiple quantum wells (MQWs) is used. That is, the absorption edge is shifted toward the longer wavelength direction by applying an electric field to the modulator, so that the absorption coefficient is changed, thus modulating a light intensity. On the other hand, in the Mach-Zendor type modulator, the Pockets effect (electrooptic effect) for a bulk semiconductor or the quantum confined stark effect for MQWs is used. That is, the infractive index is changed by applying an electric field to the modulator.
The electroabsorption optical intensity modulator can remarkably reduce the waveform chirping phenomenon, as compared with the direct modulation system by the semiconductor laser diode; however, the waveform chirping amount cannot be zero. On the other hand, in the Mach-Zendor type modulator, the waveform chirping amount can be zero in principle; however, the Mach-Zendor type modulator is complex in structure and driving method due to the interference type structure where a non-linear waveguide structure is adopted.
A prior art electroabsorption optical intensity modulator includes a semiconductor buffer, a first semiconductor cladding layer, a semiconductor optical absorption layer, a second semiconductor cladding layer and a semiconductor cap layer are formed on a semiconductor substrate. Also, a first electrode is formed on the second semiconductor cap layer, and a second electrode is formed on a second surface of the semiconductor substrate. In the prior art electroabsorption optical intensity modulator, however, a positive chirping is usually generated. In addition, if the dewavelength between an incident light and the absorption edge wavelength of the semiconductor optical absorption layer is reduced, a negative chirping operation can be carried out. In this case, however, the absorption coefficient is increased, so that a sufficient output cannot be obtained at a signal ON state. This will be explained later in detail.
Recently, in the electroabsorption optical intensity modulator a prebias applying method is adopted to reduce the waveform chirping phenomenon, thus overcoming the limit of transmission distance caused by the dispersion. In a 10 Gb/s transmission, after a definite bias voltage is applied to the modulator, an electrical signal is superposed thereon, to enhance the duration of fiber dispersion, thus overcoming the limit of dispersion of transmission distance (see: K. Yamada et al., "Low-chirp, Low-polarization Dependent Characteristics of Electroabsorption Optical Intensity Modulator with an InGaAsP Bulk" IEICE, Technical Vol. 1, p. 349, C-349, 1995). Also, in a 10 Gb/s transmission, a definite bias voltage of 1.1V is applied to the modulator integrated with a distributed feedback (DFB) laser diode, to enhance the duration of fiber dispersion, thus obtaining a transmission distance of 100 km (see: K. Morita et al., "10 Gb/s Transmission over 100 km of Standard Fiber with a Blue Chirp Modulator Integrated DFB laser", IEILE, Technical Vol. 1, p. 301, C-301, 1995). However, when a bias voltage is applied to the modulator, an outgoing light power at a signal ON state is reduced, and thus, the signal ON/OFF ratio is deteriorated. In addition, a circuit for generating such a bias voltage is required, thus making the optical system complex.